The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its significance to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an admission that such art is widely known or forms part of common general knowledge in the field.
Thickeners are known as a means of separating suspended pulps. They are commonly used in the field of mineral processing and extraction, as well as other industries such as paper production.
Known thickeners typically include a thickening tank and a feedwell. The feedwell includes a chamber having an inlet for receiving feed liquid and an outlet in fluid communication with the tank. In use, flocculation takes place in the tank, whereby pulp of higher relative density tends to settle towards the bottom of the tank, forming a bed of thickened pulps and dilute liquor of lower relative density is thereby displaced towards the top of the tank.
The feedwell is configured to reduce the turbulence of the incoming feed stream, to allow flocculants to be mixed into the liquid, and to allow sufficient residence time for reaction between flocculants and reagents before the treated liquid is discharged into the thickening tank. The configuration of the feedwell also promotes even distribution of liquid flowing from its outlet into the thickening tank.
The thickening tank may include a rotatably mounted rake. The rake is typically mounted on a centrally located drive shaft, driven by a motor and an associated gearbox. Each one of a plurality of radially extending rake arms is fixedly connected to the rake and supports an array of fixedly connected rake blades. The blades extend axially into the tank such that as the rake rotates, the blades sweep through the fluid in the tank. This sweeping shears the pulp particles, releasing trapped water, which rises to the surface through the sheared material, allowing the solids to settle into a more dense bed of thickened pulp. This process is referred to in the art as “dewatering”. The sweeping also rakes the settled solids to the centre of the tank from where they are discharged.
In contrast to its intended purpose, however, this rake in practice tends to push the whole or part of the settled material around with it, thus preventing it from performing its functions of assisting in the dewatering process and raking the settled solids to the central discharge point. This condition is referred to in the art as “donutting” and impedes the formation of the desired bed of relatively uniform, thickened pulp.
It is an object of the invention to overcome or substantially ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative.